Mass transfer and particle dissolution in liquid-gas and solid-liquid flows: Application to hydrogen production processes.

Abstract

Many environmental problems are related to the production, transformation and use of energy. Some of the concerns include, but are not limited to, acid rain, ozone depletion and climate change. Therefore, “greener” alternatives for energy production are sought. The hydrogen economy is one potential avenue to a clean energy system, and a promising option for hydrogen production is the thermochemical Cu-Cl cycle for water decomposition. When hydrogen or oxygen is produced from water splitting by electrolysis, thermochemical cycles or solar-based photocatalytic methods in water, bubble flow and vapour transfer into the gas phase occur during phase transition. This undesirable vapour transfer requires the use of more energy input to compensate for the evaporation heat requirement as well as for subsequent gas purification in the downstream unit. This work examines the thermodynamics and kinetics of vapour diffusion and entrainment for ascending bubbles in a vertical column through experimental studies for various gas production rates. Vapour entrainment is interpreted in terms of the phase transition rate and its dependence on various operating parameters such as gas bubble size, liquid depth, temperature and concentration is examined analytically and experimentally. A phase transition correlation is obtained to analyze these parameters and predict the amount of water carried to the surface of the liquid. It was determined that mass transfer is a function of Reynolds number (characterizes the flow regime), Eotvos number (characterizes bubbles’ shape) and distance the bubbles travel through liquid. ii Additionally, this work presents experimental studies of particle dynamics, dissolution and transport processes for hydrogen production via thermochemical water decomposition. The processes involve multiple steps, some consisting of multiphase reaction systems. It examines the importance of design optimization of liquid-solid phase systems and process integration of a thermochemical copper-chlorine cycle for hydrogen production. The dissolution of cuprous chloride particles in hydrochloric acid is examined in order to provide a predictive modeling method for more complex multiphase reacting systems that determine the thermodynamic equilibrium, which regulates the final state that the transport processes can approach. A ternary system consisting of cuprous chloride (CuCl), hydrochloric acid (HCl) and water exists in different variations of the Cu-Cl cycle, and the modeling of the solubility for the ternary system is essential to providing the mathematical boundary conditions for the transport processes occurring in the system. This work examines the transport processes involving the ternary system and the constituent solubilities. A semi-mechanistic model is obtained that formulates the solubility of the ternary system. It was found that the reaction kinetics is accelerated when the solution is turbulent and when the concentration of HCl is increased, until the thermodynamic limit is achieved. These findings are of importance in the integration of the electrolysis with other reactors of the Cu-Cl cycle.